* ms-obj.c (ms_obj_read_pre_biff8_obj): Unref pixbuf returned by
excel_read_IMDATA.

* ms-excel-read.c (excel_read_os2bmp): New. Handle headerless OS/2
bitmap.
(excel_read_IMDATA): Call excel_read_os2bmp to handle OS/2
BMP. Other format may be dumped to file.
(excel_read_sheet): Unref pixbuf returned by excel_read_IMDATA.